There are a wide variety of mills and milling processes that use a variety of techniques to reduce the size of particles. Media mills, such as ball mills and sand mills are particularly useful for obtaining impact between the media and the particle to be reduced in size. However, the media in media mills typically reduce the throughput of material and make it difficult to separate the crushed or ground material from the media. This is particular true with respect to malleable materials such as sawdust, switchgrass, and other cellulosic materials. Also, it is difficult to obtain particles in the submicron range with media mills of the current design.
While ball mills are suitable for dispersing particles and grinding almost anything, they were extremely noisy. Sand mills are quieter than ball mills and can disperse almost anything, but grind very few materials suitably. For example, pigments used in paints may not be ground sufficiently in sand mills. Comparatively speaking, ball mills provide 65 to 75% impact grinding while sand mills provide only 2 to 4% impact grinding with very little opposing vector grinding. “Opposing vector” means energy source or sources that are directly opposed to one another or opposed with slight angular opposition. Accordingly, paint manufacturers have found it was necessary to pre-grind pigments in jet mills. However, the natural earth oxide pigments, used mainly in paint primers cannot stand the cost of jet mill or air grinding. For illustration purposes, a comparison of milling time between a micro-ball mill and a sand mill with 4 mm shot media and 1 mm shot media is shown in FIGS. 1A and 1B where shear gives way to stress at about 7 to 10 microns.
From FIGS. 1A and 1B, it is evident that the micro-ball mills show 85% to 90% particle size reduction in 1-2 minutes for soft to very hard materials. The sand mill provided very little grinding for hard to medium hard materials. Thus jet milling is used for medium hard to hard materials. Grinding is slow in sand mills and relatively fast in micro opposing vector ball mills.
The next advance in grinding was an impact media mill that gave 90% particle reduction in the first 10% of normal grinding time. The impact media mill uses opposing energy vectors set to different degrees of interruption and disruption within the confines two media mill barrels. The barrels are designed to prevent vortex formation during grinding. The sidewalls of the barrels are used for transportation, not for spinning force contact between the particles. The chamber and discs are designed to give two thirds head-on impact and one third of angular contact rotation providing about 20% Hochberg grinding. The design of the impact media mill enabled a significant reduction in barrel wear during grinding. However, like other mills, the impact media mill is not efficient or effective for reducing particles to submicron size, i.e., less than 1 micron.
Materials such as fiberglass and fly ash act as fillers in the micron range, but may actually react in the submicron range, similar to pozzolanic materials. A “pozzolan” is defined as a siliceous or siliceous and aluminous material, which in itself possesses little or no cementing property, but will in a finely divided form—and in the presence of moisture—chemically react with calcium hydroxide at ordinary temperatures to form compounds possessing cementitious properties. Pozzolans typically have a particle size of less than 10 microns, such as in the −10 to −20 micron range. Fly ash is the most commonly known artificial pozzolan and results from the burning of pulverized coal in electric power plants. The amorphous glassy spherical particles are the active pozzolanic portion of fly ash. Class F fly ash readily reacts with lime (produced when portland cement hydrates) and alkalis to form cementitious compounds. In addition to that, Class C fly ash may also exhibit hydraulic (self-cementing) properties. Other pozzolanic materials may be obtained from rice hulls, silica fume, other aluminosilicate/silicoaluminate waste materials and ores.
The use or disposal of waste materials such as fiberglass, fly ash, silica fume, and the like is a growing problem. Accordingly, processes and methods are needed to increase the usefulness of such waste materials. One means of improving the usefulness of such materials is by grinding the materials to provide pozzolanic materials that more readily react rather than acting simply as fillers. Fly ash below 10 microns in size will react in about five days, whereas submicron size fly ash will react in 4 to 8 hours. Reaction of such materials changes the chemical nature of the materials. FIG. 2 shows the reaction times of fly ash based on the particle size of the fly ash. Fly ash becomes a 95% pozzolan and loses 90% of its porosity at 10 microns and below. Only 40 wt. % of normal fly ash reacts in 28 to 60 days, while 30 wt. % of the normal fly ash takes up to a year to react. The other 30 wt. % of normal fly ash does not react at all and acts as a filler. If the fly ash was ground in an impact mill, 60 wt. % will react in 48 hours, 30 wt. % will react in 5 days, 7 wt. % of the remaining 10% will react in 28 to 60 days. Accordingly, over 90 wt. % of the ground fly ash will react in 5 days. Similar observations can be made for US Silica 325 ground in a ball mill or impact media mill as shown in FIG. 3 and for sand as shown in FIG. 4.
A disadvantage of conventional ball mills and other media mills is that it can take 250 hours or more of grinding and multiple mill changes to obtain particles that have an average −5 micron size. Thus conventional mills cannot effectively produce particles in the −10 to −20 micron range. Ideally, the cost of −20 micron material could be cut drastically if 90 plus percent of the waste material could be ground to −20 microns in a single mill or in a mill that is devoid of media.
In view of the foregoing, embodiments of the disclosure provide a vertical mill and discs therefor for grinding particles to submicron size. The vertical mill includes two overlapping vertical barrels. Each barrel has a shaft and a plurality of circular discs attached to the shaft disposed in each barrel for grinding materials to submicron size. The discs have an open mesh design provided by overlapping rectangular blades. Each blade has at least two opposing walls orthogonal to a plane defined by the disc. Each disc on one shaft overlaps a disc on the other shaft by from 30 to 45% of the diameter of the discs. An inlet is provided for feeding material to be ground by the mill to the overlapped portion of the discs. An outlet is disposed at an end of the mill opposite the inlet for removing ground material from the mill.
Another embodiment of the disclosure provides a disc for a grinding mill for producing submicron particles. The disc has an open mesh circular disc having a plurality of overlapping rectangular blades, each blade having at least two opposing walls orthogonal to a plane defined by the disc.
A further embodiment of the disclosure provides a horizontal mill for grinding particles to submicron size. The mill includes two overlapping horizontal barrels, each barrel having a horizontal shaft and a plurality of circular discs attached to the shaft disposed in each barrel for grinding materials to submicron size. The discs have an open mesh design provided by overlapping rectangular blades, each blade having at least two opposing walls orthogonal to a plane defined by the disc, and each disc on one shaft overlaps a disc on the other shaft by from 30 to 45% of the diameter of the discs. An inlet is provided for feeding material to be ground by the mill to the discs orthogonal to an axis of rotation of the discs in the barrels. An outlet is disposed at an end of the mill opposite the inlet for removing ground material from the mill.
An advantage of the embodiments of the disclosure is that the mill may be used to grind particles to submicron size that are otherwise difficult to grind such as malleable materials. The high speed kinetics of the slicing and cutting blades of discs made of hardened stainless steel will also shatter and break friable structures and crystals. Materials that may be effectively ground to submicron size include, but are not limited to fly ash, sawdust, switchgrass, cellulosics in general, fiberglass, pigments, sand, and the like. Grinding may be done in the presence or absence of media, and may be ground more efficiently with higher throughput that with conventional media grinding mills. Unlike conventional mills, the mill and disc of the disclosed embodiments may be used to grind relatively long strand materials to provide pozzolanic materials that react rather than serve merely as filler materials in compositions such as concrete, mortar, paint, and the like. Also, an increased overlap area of the discs in the central portion of the mill may provide equal shear value without the use of media.